This invention is directed to improvements in particulate and filamentary mineral polymer-reinforcing compositions and to their preparation. More particularly, this invention relates to chemically modified mineral reinforcing agents for thermoplastic polymers.
The Prior Art: Thermoplastic polymers are finding an increasing number of uses as structural materials. They are especially attractive as replacements for metals because of the reduction in weight that can often be achieved, as for example, in automotive applications. However, for any particular application, a thermoplastic polymer by itself may not offer the combination of properties desired and means to correct this deficiency are therefore of interest. In order to increase the rigidity and strength of thermoplastic polymers, it is a common practice to incorporate a quantity of filler, e.g., a natural or synthetic mineral material, in the particulate or filamentary form, e.g., as fibers or flakes. When the mixture of polymer and fibers or flakes is injection molded into a sheet form, the flow tends to cause the particles of filler to line up parallel to the sheet. If the particles have a high aspect ratio and have a high rigidity and strength, they will then constitute an effective reinforcement in the direction of alignment.
Several types of mineral fillers are in commercial use. The most frequently employed are glass fibers, asbestos fibers, clay-type minerals such as kaolin, calcium salts such as wollastonite and calcium carbonate and platy clay minerals such as talc and mica.
It is known that glass filaments must receive a chemical surface treatment or "sizing" in order to be effective as polymer reinforcement. Silicon compounds, such as polysiloxanes, are typically employed for this purpose to provide adhesion between the glass and the thermoplastic polymer. Other agents, such as "starch oil", provide lubrication; polymeric materials have been used to bind the fibers into a bundle.
In the case of normal sizing of glass fibers, the sizing compounds are not covalently bonded to the matrix. In such systems the glass fiber has a corrosion layer on the surface. This is a layer of etched glass which has had the alkali earth oxides leached out of it by water. On the surface of this corrosion layer there are islands of polysiloxanes deposited by the silane coupling agent. These islands of polysiloxanes are hydrogen bonded to the corrosion layer and not directly covalently bound to the matrix. It is well known that glass laminates treated with silane sizing agents lose strength on immersion in water. This is because the water diffuses along the surface of the glass fiber through this corrosion layer, wets the corrosion layer and lubricates the interface between the polysiloxane surface coat and the glass. This again demonstrates that the siloxanes are not directly bonded to the glass.
Another common surface finish for glass fibers is a family of chromium complexes, known as volanes. These have an ionic interaction with the surface of the mineral. It is not clear whether they are in fact ionically bonded, as is claimed for them, or whether they are hydrogen bonded like the silane compounds. In any event, they are not covalently bonded.
A variety of similar treatments have also been disclosed for mineral fillers other than glass fibers, especially mica and wollastonite. For example, it has been suggested to polymerize monomers such as methyl methacrylate, acrylonitrile or the like by a free radical mechanism to deposit a polymer on the mineral surface. On the basis of known reactivity of the different sorts of radicals it is expected that these polymers are not covalently bonded to the mineral surface.
Mineral fillers of the prior art, especially those other than glass fibers, must be added in large concentrations, typically from 20 to 40 parts by weight per hundred parts of resin (phr) or more, in order to achieve the desired increase in stiffness. Addition of such large amounts of mineral fillers causes losses in other properties of the polymers, primarily impact resistance and tensile properties, rendering such polymers unsuitable for many premium uses.
Appropriately selected reinforcing materials of the present invention have the advantage, when used in lower concentrations from 1 to 20 phr, of producing a desired increase in stiffness of polymers without significant loss in other desired properties, and even a gain in impact strength in some cases.